During the manufacturing of certain silicon semiconductor devices such as MOS type semiconductor devices, the amount of certain contaminants such as heavy metals or particles which may be present when the semiconductor wafer is formed can seriously degrade the performance of the final devices. Despite careful manufacturing procedures, there is a possibility that contaminants may appear in the wafer and may be sufficient to make any devices made from the contaminated wafer useless.
The industry has recognized this problem and it is the accepted practice in the industry to test silicon semiconductor wafers for contaminants after a typical batch of up to 25 or more have been made and have been partially processed. One of the wafers in the batch is used for tests and is then discarded. The remaining wafers are further processed only if the tested wafer provides satisfactory data.
The contaminants in a silicon semiconductor wafer are present on or in the silicon substrate prior to gate oxidation which is one of the production operations. The contaminants become or tend to become incorporated into the silicon oxide as it is being formed. Typically, the silicon oxide layer is about 500 Angstroms thick and this layer will decrease in thickness during additional operations.
The test for contaminants in a silicon semiconductor wafer according to the prior art is carried out on the test wafer after the batch of wafers including the test wafer has been subjected to gate oxidation. The test wafer is separated from the batch and is subjected to hydrofluoric acid fumes (HF) in a chamber to etch away the oxide layer. The movement of the test wafer is performed by an operator, thereby increasing the possibility of further contamination. Thereafter, the procedure is to perform a "liquid drop test". As used herein, the "liquid drop test" refers to a test sometimes referred to in the industry as the "Toshiba Process" in which a predetermined quantity of a liquid solution usually comprising hydrofluoric acid and certain other components (hereinafter, the "test drop") is deposited on the test wafer on the side from which the oxide layer had been removed and the test wafer is manually manipulated to cause the movement of the test drop relative the surface of the test wafer so that the test drop contacts substantially every portion of the surface of the test wafer on that side.
The test drop is removed by pouring it into a cup and subjected to tests to determine the presence of contaminants so that a decision can be made as to the entire batch. The specific components of the test drop are usually regarded by companies as proprietary and these components can vary depending on the specific manufacturing process and the manufacturing materials being utilized.
The repeatability and reliability of the prior art procedure for the liquid drop test depends entirely on the mechanical skill of the person moving the test drop over the surface of the test wafer. Preferably, the test drop should contact every portion of the surface and the contact time at each portion should be approximately the same. The total time required by the manual manipulation of the test drop over the surface of the test wafer must be sufficient to provide assurance that the test drop has been maneuvered over the entire surface. Thus, any attempt to minimize the time for this procedure creates a risk of an unreliable movement of the test drop over the surface of the test wafer as well as unequal time of the test drop over the surface of the test wafer.
It can be seen that a need exists for a reliable and repeatable method and system for conducting the liquid drop test for silicon semiconductor wafers. In addition, there is a great need for a semi-automated and preferably a fully automated system for moving a test wafer into a testing position, conducting the liquid drop test, and thereafter, removing and analyzing the liquid drop for the presence of contaminants.